encoding/asn1: add NullBytes and NullRawValue for working with ASN.1 NULL

There were a number of places in crypto/x509 that used hardcoded
representations of the ASN.1 NULL type, in both byte slice and
RawValue struct forms. This change adds two new exported vars to
the asn1 package for working with ASN.1 NULL in both its forms, and
converts all usages from the x509 package.

In addition, tests were added to exercise Marshal and Unmarshal on
both vars.

See #19446 for discussion.

Change-Id: I63dbd0835841ccbc810bd6ec794360a84e933f1e
Reviewed-on: https://go-review.googlesource.com/38660
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/x509/x509.go

@@ -69,9 +69,7 @@ func marshalPublicKey(pub interface{}) (publicKeyBytes []byte, publicKeyAlgorith
 		publicKeyAlgorithm.Algorithm = oidPublicKeyRSA
 		// This is a NULL parameters value which is required by
 		// https://tools.ietf.org/html/rfc3279#section-2.3.1.
-		publicKeyAlgorithm.Parameters = asn1.RawValue{
-			Tag: 5,
-		}
+		publicKeyAlgorithm.Parameters = asn1.NullRawValue
 	case *ecdsa.PublicKey:
 		publicKeyBytes = elliptic.Marshal(pub.Curve, pub.X, pub.Y)
 		oid, ok := oidFromNamedCurve(pub.Curve)
@@ -355,10 +353,8 @@ func rsaPSSParameters(hashFunc crypto.Hash) asn1.RawValue {
 
 	params := pssParameters{
 		Hash: pkix.AlgorithmIdentifier{
-			Algorithm: hashOID,
-			Parameters: asn1.RawValue{
-				Tag: 5, /* ASN.1 NULL */
-			},
+			Algorithm:  hashOID,
+			Parameters: asn1.NullRawValue,
 		},
 		MGF: pkix.AlgorithmIdentifier{
 			Algorithm: oidMGF1,
@@ -368,10 +364,8 @@ func rsaPSSParameters(hashFunc crypto.Hash) asn1.RawValue {
 	}
 
 	mgf1Params := pkix.AlgorithmIdentifier{
-		Algorithm: hashOID,
-		Parameters: asn1.RawValue{
-			Tag: 5, /* ASN.1 NULL */
-		},
+		Algorithm:  hashOID,
+		Parameters: asn1.NullRawValue,
 	}
 
 	var err error
@@ -418,11 +412,10 @@ func getSignatureAlgorithmFromAI(ai pkix.AlgorithmIdentifier) SignatureAlgorithm
 	// https://tools.ietf.org/html/rfc3447#section-8.1), that the
 	// salt length matches the hash length, and that the trailer
 	// field has the default value.
-	asn1NULL := []byte{0x05, 0x00}
-	if !bytes.Equal(params.Hash.Parameters.FullBytes, asn1NULL) ||
+	if !bytes.Equal(params.Hash.Parameters.FullBytes, asn1.NullBytes) ||
 		!params.MGF.Algorithm.Equal(oidMGF1) ||
 		!mgf1HashFunc.Algorithm.Equal(params.Hash.Algorithm) ||
-		!bytes.Equal(mgf1HashFunc.Parameters.FullBytes, asn1NULL) ||
+		!bytes.Equal(mgf1HashFunc.Parameters.FullBytes, asn1.NullBytes) ||
 		params.TrailerField != 1 {
 		return UnknownSignatureAlgorithm
 	}
@@ -928,16 +921,13 @@ type distributionPointName struct {
 	RelativeName pkix.RDNSequence `asn1:"optional,tag:1"`
 }
 
-// asn1Null is the ASN.1 encoding of a NULL value.
-var asn1Null = []byte{5, 0}
-
 func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
 	asn1Data := keyData.PublicKey.RightAlign()
 	switch algo {
 	case RSA:
 		// RSA public keys must have a NULL in the parameters
 		// (https://tools.ietf.org/html/rfc3279#section-2.3.1).
-		if !bytes.Equal(keyData.Algorithm.Parameters.FullBytes, asn1Null) {
+		if !bytes.Equal(keyData.Algorithm.Parameters.FullBytes, asn1.NullBytes) {
 			return nil, errors.New("x509: RSA key missing NULL parameters")
 		}
 
@@ -1650,9 +1640,7 @@ func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgori
 		pubType = RSA
 		hashFunc = crypto.SHA256
 		sigAlgo.Algorithm = oidSignatureSHA256WithRSA
-		sigAlgo.Parameters = asn1.RawValue{
-			Tag: 5,
-		}
+		sigAlgo.Parameters = asn1.NullRawValue
 
 	case *ecdsa.PublicKey:
 		pubType = ECDSA

src/encoding/asn1/asn1.go

@@ -207,6 +207,14 @@ func parseBitString(bytes []byte) (ret BitString, err error) {
 	return
 }
 
+// NULL
+
+// NullRawValue is a RawValue with its Tag set to the ASN.1 NULL type tag (5).
+var NullRawValue = RawValue{Tag: TagNull}
+
+// NullBytes contains bytes representing the DER-encoded ASN.1 NULL type.
+var NullBytes = []byte{TagNull, 0}
+
 // OBJECT IDENTIFIER
 
 // An ObjectIdentifier represents an ASN.1 OBJECT IDENTIFIER.

src/encoding/asn1/asn1_test.go

@@ -479,6 +479,7 @@ var unmarshalTestData = []struct {
 	out interface{}
 }{
 	{[]byte{0x02, 0x01, 0x42}, newInt(0x42)},
+	{[]byte{0x05, 0x00}, &RawValue{0, 5, false, []byte{}, []byte{0x05, 0x00}}},
 	{[]byte{0x30, 0x08, 0x06, 0x06, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d}, &TestObjectIdentifierStruct{[]int{1, 2, 840, 113549}}},
 	{[]byte{0x03, 0x04, 0x06, 0x6e, 0x5d, 0xc0}, &BitString{[]byte{110, 93, 192}, 18}},
 	{[]byte{0x30, 0x09, 0x02, 0x01, 0x01, 0x02, 0x01, 0x02, 0x02, 0x01, 0x03}, &[]int{1, 2, 3}},
@@ -1007,3 +1008,28 @@ func TestUnexportedStructField(t *testing.T) {
 		t.Errorf("got %v, want %v", err, want)
 	}
 }
+
+func TestNull(t *testing.T) {
+	marshaled, err := Marshal(NullRawValue)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(NullBytes, marshaled) {
+		t.Errorf("Expected Marshal of NullRawValue to yeild %x, got %x", NullBytes, marshaled)
+	}
+
+	unmarshaled := RawValue{}
+	if _, err := Unmarshal(NullBytes, &unmarshaled); err != nil {
+		t.Fatal(err)
+	}
+
+	unmarshaled.FullBytes = NullRawValue.FullBytes
+	if len(unmarshaled.Bytes) == 0 {
+		// DeepEqual considers a nil slice and an empty slice to be different.
+		unmarshaled.Bytes = NullRawValue.Bytes
+	}
+
+	if !reflect.DeepEqual(NullRawValue, unmarshaled) {
+		t.Errorf("Expected Unmarshal of NullBytes to yield %v, got %v", NullRawValue, unmarshaled)
+	}
+}

src/encoding/asn1/common.go

@@ -24,6 +24,7 @@ const (
 	TagInteger         = 2
 	TagBitString       = 3
 	TagOctetString     = 4
+	TagNull            = 5
 	TagOID             = 6
 	TagEnum            = 10
 	TagUTF8String      = 12